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Best practice for accurate PH readings?

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Hey there. I'm puzzled.
I tested perims on all 3 of my tanks this morning. Had an odd thing happen with my PH readings. I tested PH last so the water sat a bit longer in my samples. 1 sample read exactly as I thought it would: 7.2 The other two, however, one didn't even register and the other was barely a 6. So I retested all 3 and came up with the same numbers. Then I drew fresh water from the two tanks in question, tested right away and came up with 7.2 - as I'd hoped, because that's what it usually runs in all 3 tanks.

So -- now I wonder if I should trust my test kit or if it makes a difference (in some cases) if you leave the water set a bit longer. Most time elapsed would have been 15 minutes. The first sample that read 0 was in a paper cup. the other that read only 6 was in a plastic cup.

Liters to Gallons conversion calculator
"Keeping fish for any period of time doesn't make you experienced if you're doing it wrong. What does, is acknowledging those mistakes and learning from them." ~Aeonflame
"your argument is invalid." ~Mommy1

Draw a water sample from the tank with a clean cup. Make sure you rinse out the pH test tube with water from the tank 3 or 4 times. Now fill the test tube and test for pH. Make sure to shake to pH test bottle for 5 - 10 minutes before adding drops to the test tube. Rinse the test tube with water from the cup before you test again.

Follow the same procedure with water out of the tap making sure you rinse the test tube before and after each test.

Do your fish appear to be in any kind of distress...showing signs of stress?

If not, I wouldn't worry about pH because you change water in your tank often enough so that the pH shouldn't vary that greatly. Might be time for a new test kit too.

appreciate the reply. The test instructions do no specify so this is good info. So ... I retested the tank that initially read 7.2 after the water had sat. It dropped to 6.6 with an immediate read. Find it interesting that the other 2 tanks rose with new samples but this one dropped.
Apparently, the test could be inaccurate + or - if the water is not tested right away.

Follow up - would 'testing right away' also hold true for A/N/N? could the readings change on those tests too if the water isn't checked as soon as the sample is taken? I'm thinking not since some folks take their samples to the LFS for testing??
again, thanks!

Do your fish appear to be in any kind of distress...showing signs of stress?If not, I wouldn't worry about pH because you change water in your tank often enough so that the pH shouldn't vary that greatly. Might be time for a new test kit too.

Fish are fine. I was just curious as the test results puzzled me
thanks